Preheat your oven to 325°F and line a 9x13 inch baking pan with parchment paper, allowing it to extend up the sides for easy removal later. While the oven heats, whisk together the flour, cornstarch, baking powder, and salt in a medium bowl. Set this dry mixture aside—having it ready will speed up assembly once you start working with the wet ingredients.

Combine the milk and apple cider vinegar in a small bowl and let it sit for 5-8 minutes. This creates a vegan buttermilk substitute that will add tanginess and help activate the baking powder, giving your brownies a better texture and subtle depth. Don't skip the resting time—it matters for the chemical reaction.

In a large bowl, melt the butter over medium heat (or use the microwave in 30-second intervals), then remove from heat and stir in the brown sugar and sugar for 2-3 minutes until well combined. Add the vegan milk mixture from Step 2 and microwave the entire mixture for 2 minutes to help dissolve the sugars completely. Whisk vigorously for 1-2 minutes until the mixture is smooth and the sugar is fully incorporated—this helps create a fudgy texture. Sift the cocoa powder directly into the mixture and stir for about 2 minutes until there are no lumps and the color is even. The chocolate should be fully hydrated and silky. Stir in the vanilla extract and espresso powder, which will deepen the chocolate flavor without making the brownies taste like coffee.

Add the dry ingredient mixture from Step 1 to the chocolate batter from Step 3 and stir until just combined—don't overmix, as this can develop too much gluten and make the brownies tough. The batter should come together with a few streaks of flour remaining is fine. I like to fold rather than stir at this point, using a gentle hand to incorporate everything while keeping the batter light and fudgy.

Gently fold the chopped chocolate pieces and most of the crushed Oreo chunks (reserve about 1/4 cup for topping) into the batter using a rubber spatula. Pour the batter into your prepared pan, smoothing the top with a spatula, then scatter the reserved Oreos evenly across the surface, pressing them down slightly so they stay on top during baking.

Bake for 40-50 minutes at 325°F, until the edges are set but the center still jiggles slightly when you gently shake the pan—this is key for fudgy brownies. The baking time will vary depending on your oven, so start checking around 40 minutes. Let the brownies cool in the pan for at least 15 minutes before cutting into squares. I find that cooling them completely (or even chilling them in the refrigerator for an hour) makes them much easier to cut cleanly and improves the texture.